Mark Phelan: The faster you go, the less m.p.g. you get

January 27, 2013

Opel's Cascada convertible should be one of the handsomest cars debuting at the Geneva, Switzerland, auto show in March, but don't look for it on U.S. roads any time soon. / Axel Wierdemann/Special to the Free Press

The Cascada, a convertible with a high-tech 1.6-liter turbo engine and soft-top that opens and closes at up to 30 m.p.h., could be in Buick's future. / Axel Wierdemann/Special to the Free Press

How much does your fuel economy fall -- and how much does your fuel cost rise? -- if you drive at 60 m.p.h. rather than 50? How about 70 m.p.h.? 80?

Would you believe a 41% decrease in fuel economy from 50 m.p.h. to 80? That's like paying $1.38 more per gallon of gasoline, according to the U.S. Department of Energy's website Fueleconomy.gov.

Until last week, nobody had quantified the effect real-world speeds have on fuel economy, despite the fact that most people hit those speeds on the highway every day. New research by the DOE --which works with the EPA to generate window-sticker mileage figures -- should help create models to help drivers figure out the effect higher speeds have on their specific vehicle.

The differences from one vehicle to another are surprising, according to Brian West, a development engineer who worked on the study at DOE's Oak Ridge National Laboratory in Tennessee.

The smallest drop in fuel efficiency was 6.9% from 50 to 60 m.p.h., while one vehicle lost a surprising 26% between 70 and 80 m.p.h. Other vehicles saw their fuel economy decrease as much as 18.3% at 60 and as little as 10.8% at 80.

There wasn't much correlation between decreasing fuel economy and the vehicles' frontal area and aerodynamic drag. That's one of the surprises the DOE team is still trying to figure out, because it seems to violate a law of physics.

The study compares 74 vehicles' fuel economy at 50, 60, 70 and 80 m.p.h. All the vehicles were tested on chassis dynamometers using Society of Automotive Engineers standards. DOE tested 24 of the vehicles in Oak Ridge while Chrysler provided data from 50 vehicles it tested at its labs in Michigan. Vehicles of all body styles, drivetrains and major manufacturers were represented.

While the figures will vary from one vehicle to the next, West said you can use the average decreases for rule-of-thumb estimates of speed vs. fuel economy. You can assume a 12.4% decrease at 60 vs. 50 m.p.h., 14% at 70 vs. 60 m.p.h. and 15.4% more at 80 m.p.h. Thus, 41.8% lower at 80 vs. 50.

Then you can figure out exactly how much more it will cost you to drive to Chicago at 75 m.p.h. than 50, and whether the money you save at the slower speed is worth nearly two hours longer on the road.

New Opel convertible looks great, but not for U.S.

Opel's sleek Cascada midsize convertible should be one of the handsomest cars debuting at the Geneva, Switzerland, auto show in March, but don't look for it on U.S. roads any time soon.

Opel and Buick have aligned their design and product lines, but some models that make sense for Opel don't fit Buick yet, and may never.

The Cascada is a prime example. Its styling would look natural on the showroom floor between the Verano and Regal, but its price is out of line with Americans' view of Buick. Cascada stickers will start at the equivalent of $34,922.

That's nearly $17,500 less than the Audi A5 convertible. Buick is a long way from facing off against high-end Audis in the U.S.

If GM's plan to restore Buick to luxury status works, a convertible with a high-tech 1.6-liter turbo engine and soft-top that opens and closes at up to 30 m.p.h. could be in the brand's future. But not its immediate future.

BMW borrows from Volt

BMW will use a range-extending gasoline engine to help its upcoming i3 electric car appeal to more buyers, Automotive News reports.

That's the same approach GM took with the Chevrolet Volt. A range-extending engine that generates more electricity can help electric cars overcome their biggest disadvantages: short range and long charging time.

BMW's R&D boss said up to 50% of initial i3 buyers could choose the extended-range model, though he expects that to fall to 20% over time.